1. Field of the Invention
This invention relates to resin boards for use in wiring boards, wiring boards formed from the resin boards, and a method of manufacturing the wiring boards.
2. Description of the Related Art
Resin is finding remarkably increased use in the fields where metal is conventionally used, owing to its features, such as the ease with which resin can be shaped, a high degree of flexibility in characteristic values, such as strength, and lightness in weight. However, resin also has drawbacks, such as the absence of electrical conductivity and low hardness, and therefore, it has been proposed to combine resin with metal, or the like, so as to eliminate these drawbacks.
As an example of method for providing resin with electrical conductivity, powder of a conductive metal or carbon fibers, for example, is mixed into the resin. However, a large amount of conductive metal needs to be added to the resin so as to impart sufficiently high conductivity to the resin, which may cause adverse influences on the physical properties and increased cost. Thus, another method is known in which a metal film, or a film of a conductive oxide, such as ITO, is formed on a surface of the resin. The conductive film may be formed by a physical method, such as vapor deposition or sputtering, or a chemical method, such as electroless plating. The physical method generally requires large-sized equipment, such as a vacuum tank, and thus suffers from significant restrictions in terms of space or productivity, which undesirably results in increased cost.
In the case where a film of metal is formed on a resin surface by electroless plating, the strength of adhesion between the metal film and the resin is low, and the metal film is likely to peel off from the resin. In view of this problem, the following steps are generally carried out: chemical etching is performed on a resin material so as to make its surface rough, and the etched resin material is then subjected to electroless plating. However, the method that makes the resin surface rough by etching causes a reduction of the surface smoothness, and requires the use of a poisonous or deleterious substance, such as chromic acid, permanganic acid, or sulfuric acid, giving rise to a problem concerned with treatment of waste liquid.
With this being the situation, Japanese Laid-open Patent Publication No. 2002-309377 (JP-A-2002-309377) discloses a method in which a resin material is brought into contact with an ozone solution and is then treated with a solution containing a surface-active agent or surfactant and an alkaline component, and then electroless plating is carried out. According to this method, the cleavage of double bonds on the surface of the resin material takes place due to oxidation caused by ozone, and polar groups are produced on the resin surface. Also, the alkaline component removes an embrittled layer of the resin material, and the surface-active agent is adsorbed by the polar groups. During catalyst treatment prior to electroless plating, a catalyst is adsorbed by the surface-active agent adsorbed on the polar groups. Therefore, metal is likely to be bonded with the polar groups during electroless plating, and the adhesion strength of the resulting electroless-plating film is improved.
JP-A-2005-042029 also proposes a resin board having a resin-metal composite layer, which consists of a resin substrate and the resin-metal composite layer formed integrally on a surface of the resin substrate and having fine metal particles uniformly dispersed in a resin matrix, and a method of manufacturing the resin board.
The resin-metal composite layer of the resin board imparts some characteristics, such as electrical conductivity, wear resistance, light resistance and flame retardancy, to the resin board, and the resin-metal composite layer can be made transparent or translucent. Therefore, the resin board having the resin-metal composite layer can be used in various applications, such as liquid crystal displays and electronic circuit boards. According to the manufacturing method disclosed in JP-A-2005-042029, the resin-metal composite layer can be easily formed without requiring equipment like a vacuum tank, and therefore the resin board can be manufactured in a short time with a reduced number of steps.
In a typical wiring board, a spacing of 100 μm or larger is provided between adjacent wires on the board. In a wiring board of a small-sized high-density component, however, the spacing between adjacent wires on the board needs to be 100 μm or smaller.
In the case where a resin-metal composite layer is formed using the technology disclosed in JP-A-2005-042029, and wires are formed with small spacings (of, for example, 100 μn or less) on the surface of the composite layer by electroless plating, portions of the resin-metal composite layer located between the wires need to be removed by etching in a later process step.
In the meantime, the technology disclosed in JP-A-2005-042029 has a problem that the adhesion strength of the plating film is low if the thickness of the resin-metal composite layer is small, more specifically, is about 20-200 nm. It is thus necessary to make the thickness of the metal-composite layer greater than 200 nm. It is, however, to be noted that the plating film is embedded in the resin after electroless plating. Thus, if a resin-metal composite layer having a thickness greater than 200 nm is formed, it is difficult to completely remove the plating film and resin-metal composite layer between wires even with etching, causing a problem of insufficient or faulty insulation.